The present invention relates to a method of treating a reverse osmosis membrane element used for reducing a solute concentration in a solution, and a reverse osmosis membrane module having the thus treated reverse osmosis membrane element.
Some reverse osmosis membranes (RO membranes) according to conventional technology have excellent performance in desalination, water permeability and separation of ionic substances. However, the conventional reverse osmosis membranes cannot provide sufficient rejection of non-electrolytic organic materials such as isopropyl alcohol (IPA) or substances (e.g., boron) that will not be dissociated in a neutral region.
Moreover, performance (e.g., salt rejection) of a conventional reverse osmosis membrane will deteriorate because of change in performance over time or swelling of the reverse osmosis membrane, which is caused by chemicals used for cleaning of the membrane. For temporarily recovering the salt rejection, the reverse osmosis membrane element is contacted with an aqueous solution of a polymer such as polyvinyl methyl ether, tannic acid or the like so as to adsorb the polymer in the surface of the reverse osmosis membrane or clog the surface of the reverse osmosis membrane with the polymer.
However, since the polymers are physically adsorbed or clogged in the surface of the reverse osmosis membrane in the conventional technique, the polymers will be desorbed from the membrane surface over time. Alternatively, the polymers can be desorbed due to chemicals used for cleaning of the membrane. Therefore, it is difficult to maintain the performance of the reverse osmosis membrane element for a long time by treating the membrane according to the conventional technique.
In a process of distillation of seawater by use of such a reverse osmosis membrane, boron is difficult to remove. Recently in Japan, a standard for inspection of distillation facilities was changed. Under this new standard, it is required that boron in seawater is reduced to 1.0 mg/L or less. This target value of 1.0 mg/L cannot be achieved necessarily by using the conventional reverse osmosis membranes, depending on the operation conditions or due to the change in performance of the reverse osmosis membranes.
An object of the present invention is to provide a method of treating a reverse osmosis membrane in order to maintain for a long time an effect of reducing a solute concentration in a solution. The reverse osmosis membrane will reject even non-electrolytic organic materials or substances such as boron that will not be dissociated in a neutral region.
According to the present invention, a concentration of a solute in a solution can be reduced by operating a membrane separator equipped with a reverse osmosis membrane element having a polyamide skin layer, in which the reverse osmosis membrane element is contacted with a bromide-containing aqueous solution of free chlorine so as to introduce the bromide into the polyamide skin layer.
Specifically in the present invention, a reverse osmosis membrane element is loaded in a pressure vessel in the membrane separator, and then contacted with a bromide-containing aqueous solution of free chlorine.
A reverse osmosis membrane module according to the present invention is equipped with the above-described reverse osmosis membrane element.
In the present invention, a reverse osmosis membrane element has a reverse osmosis membrane and also a raw-water channel provided adjacent to the reverse osmosis membrane. A flat reverse osmosis membrane will be formed like a bag and a channel for passing permeated water is provided inside thereof. The reverse osmosis membrane is wound around the outer surface of a perforated hollow tube so that the interior of the reverse osmosis membrane is connected at one end to the perforated hollow tube.
A reverse osmosis membrane module has a pressure vessel containing one or plural reverse osmosis membrane(s). The reverse osmosis membrane element is required for applying a raw-water pressure so as to carry out separation using the reverse osmosis membrane.
A membrane separator is an apparatus having a controller, a pump, a measuring instrument, a pipe, a bulb, a tank, and the like for operating the reverse osmosis membrane module.